1. Field of the Invention (Technical Field):
The present invention relates to optical wavefront sensors, to optical Fourier techniques, and to moirxc3xa9 deflectogram.
2. Background Art:
Typical analyses of moirxc3xa9 deflectograms include fringe recognition, computer Fourier transformations, and phase-shifting techniques, requiring multiple shots in order to get high resolution. Computer-based Fourier-transform and phase-shifting techniques are only with difficulty able to resolve and determine where fringes are, especially when fringes close into loops or branch.
Related wavefront sensor or Fourier transform technologies are disclosed in the following references: U.S. Pat. No. 6,130,419, to Neal, entitled xe2x80x9cFixed Mount Wavefront Sensorxe2x80x9d; U.S. Pat. No. 5,936,720, to Neal et al., entitled xe2x80x9cBeam Characterization by Wavefront Sensorxe2x80x9d; U.S. Pat. No. 5,872,648, to Sanchez et al., entitled xe2x80x9cOn-Axis Spatial Light Modulators and Methods of Usexe2x80x9d; U.S. Pat. No. 5,479,257, to Hashimoto, entitled xe2x80x9cMethod of and Apparatus for Detecting Object Position Using a Fourier Transform of the Object Image and Processing System Using the Samexe2x80x9d; U.S. Pat. No. 5,229,889, to Kittell, entitled xe2x80x9cSimple Adaptive Optical Systemxe2x80x9d; U.S. Pat. No. 5,192,982, to Lapucci, entitled xe2x80x9cCoded-Fringe Interferometric Method and Device for Wavefront Detection in Opticsxe2x80x9d; U.S. Pat. No. 5,159,474, to Franke et al., entitled xe2x80x9cTransform Optical Processing Systemxe2x80x9d: U.S. Pat. No. 5,111,314, to Leib, entitled xe2x80x9cOptical Correlator Interconnect for Optical Computerxe2x80x9d; U.S. Pat. No. 5,090,795, to O""Meara et al., entitled xe2x80x9cIntegrated Adaptive optics Apparatusxe2x80x9d; U.S. Pat. No. 5,042,950, to Salmon, Jr., entitled xe2x80x9cApparatus and Method for Laser Beam Diagnosisxe2x80x9d: U.S. Pat. No. 4,872,135, to Peterson et al., entitled xe2x80x9cDouble Pinhole Spatial Phase Correlator Apparatusxe2x80x9d; U.S. Pat. No. 4,798,437, to Rediker et al., entitled xe2x80x9cMethod and Apparatus for Processing Analog Optical Wave Signalsxe2x80x9d; U.S. Pat. No. 4,351,589, to Chavel et al., entitled xe2x80x9cMethod and Apparatus for Optical Computing and Logic Processing by Mapping of Input Optical Intensity into Position of an Optical Imagexe2x80x9d; U.S. Pat. No. 4,187,000, to Constant, entitled xe2x80x9cAddressable Optical Computer and Filterxe2x80x9d; and U.S. Pat. No. 3,921,080, to Hardy, entitled xe2x80x9cAnalog Data Processorxe2x80x9d.
The present invention uses moirxc3xa9 deflectometry and the Fourier transforming properties of a lens to optically compute the wavefront curvature of incident light, and so computation takes place as fast and as simply as possible. Optical Fourier processing is automatically performed within the optical device, and so information is available literally at the speed of light. The deflectograms are obtained under a set of conditions whereby they do not have to be digitized, and the fringes do not have to be resolved by detection equipment. Hence, resolution is determined by pixel size, not fringe spacing or algorithm matrix size.
Prior techniques have certain advantages. For example, the Schlieren technique, discussed in E. Hecht, Optics (2d ed. 1987), gives the magnitude of wavefront distortion, but not the sense of curvature. Shack-Hartmann sensors (see, e.g., U.S. Pat. No. 5,936,720) are very popular for wavefront correction, but suffer from trade-offs between diffractive effects and sensitivity, dynamic range, and sensitive alignment issues.
However, the present invention overcomes the technical difficulties of previous techniques at a minimal cost. Devices of the invention can be housed much like camera lenses are, permitting parameters varying sensitivity and dynamic range, such as relative gratings angle and grating separation, to be easily adjusted and measured.
The present invention is of an apparatus and method for wavefront sensing comprising: employing two moirxc3xa9 gratings in an optical path; optically Fourier transforming a moirxc3xa9 deflectogram produced by the gratings; variably transmitting the transformed moirxc3xa9 deflectogram; and receiving an image of the variably transmitted and transformed moirxc3xa9 deflectogram. In the preferred embodiment, the variable transmission is accomplished by transmission filter, most preferably a transmissive optic encoding intensity information upon the moirxc3xa9 deflectogram as a function of fringe angle. For example, the function can be a triangular transmission function centered on the (0,0) order spatial frequency spot and oriented at 45 degrees to the y-axis. The optical Fourier transform is preferably accomplished by a lens and the variable transmission by an apodized slit.
The invention is also of a Fourier moirxc3xa9 generating apparatus and method for wavefront sensing comprising: employing two moirxc3xa9 gratings in an optical path: employing an optical Fourier transform means following the gratings in the optical path; and employing an apodized optical means following the lens in the optical path. In the preferred embodiment, the apodized optical means is an apodized slit encoding intensity information upon the moirxc3xa9 deflectogram as a function of fringe angle, and the transform means is a lens.
A primary object of the present invention is to provide for rapid, high-resolution, computer-free, quantitative, and inexpensive wavefront measurement.
A primary advantage of the present invention is that optical moirxc3xa9 and Fourier techniques are the main processing tools and so the device of the invention operates at the speed of light
Another advantage of the present invention is that optical Fourier filtering transfers the useful information of the deflectogram (changes in fringe direction) directly to electrical signal. Certain types of morphable membrane mirrors for adaptive optics application can be driven directly from such data, obviating the need for computer-driven controls.
A further advantage is that the invention provides valuable advantages over prior wavefront detectors such as Shack-Hartmann sensors. These sensors are limited in their dynamic range because they rely on unambiguous pixel areas allocated to each lenslet in a lenslet array. To allow for unexpectedly large aberrations, the only choice is to retrofit a new lenslet array with larger lenses. The sensitivity of detection is adjustable by merely changing the distance between two gratings (ronchi rulings). This overcome another deficiency of the Shack Hartmann sensors in that these require expensive retrofitting to change sensitivity.
Yet another advantage is that the invention determines wavefront shape to a constant, i.e., the overall tilt of the wavefront remains ambiguous. Generally, this overall tilt is not considered important, as long as the shape of the wave is determined. Hence, in the present invention, the measurement is relatively Insensitive to the alignment of the beam into the device. However, tilt is precisely what is measured (not curvature) with the Shack-Hartmann device, and hence, beam alignment is a tricky issue. A beam which xe2x80x9cwandersxe2x80x9d is, then, most easily examined by the present invention.
Another advantage is that the invention has at least double the optical resolution of other moirxc3xa9 techniques because the pixel spacing of the camera determines the resolution whereas in other techniques, for computation of the data to be performed, fringes have to be resolved in the image. Because it requires two pixels to resolve a single fringe (invoking the Nyquist theorem), the increase in resolution is at least twofold.
Yet another advantage is that the invention is implementable in a very compact and low cost form.